I have a cp1h-y20dt-d (sinking output) plc supposedly to be wired to a servo controller. The pulse control method is Pulse/Direction. I've did some manual research on the PLC and found the following diagram:

For PULSE output 0 using PULSE plus DIRECTION configuration you connect CW0+ [Pulse] to the pulse input of your servo and CW1+ [Direction] to the direction input of your servo. Then connect COM[-] to both CW0- and CW1- terminals.

Pulse/Direction outputs 0 and 1 are Line Driver Outputs rated up to 1MHz.
Pulse/Direction outputs 2 and 3 are Sinking Outputs rated up to 1kHz.
If pulse/direction 2 and 3 are NOT configured in the PLC Setup then they may be used as "Normal" ON/OFF outputs in the program.

Pulse/Direction outputs 0 and 1 are Line Driver Outputs rated up to 1MHz.
Pulse/Direction outputs 2 and 3 are Sinking Outputs rated up to 1kHz.
If pulse/direction 2 and 3 are NOT configured in the PLC Setup then they may be used as "Normal" ON/OFF outputs in the program.

For Outputs 0 and 1 (Line Driver Outputs), I noticed there is no COM to connect for a sinking output, if it were to be connected to a servo controller, it should be a direct connection? As in:

Using Line Driver pulse outputs 0 and/or 1 or Sinking pulse outputs 2 and/or 3 is completely dependent on the pulse input specifications of your servo controller.
You don't specify the manufacturer/model # of the servo you intend to use so it's hard to recommend which ones you should use.
May also be worth checking with OMRON whether the Line Driver pulse outputs will work in a sinking mode. You can always test this out if you have the physical hardware available.

I see. Based on my servo's manual, the image of the internal circuit shows an open collector type input. Isit possible to link Line Driver Output to an open collector pulse input? Image below shows the open collector type input:

That all depends on your servo model input specifications.
AGAIN you haven't specified the servo model so Contact your servo supplier for more specific info.
If I were doing this I would just hookup the physical components and test if it works.